In an interior permanent magnet synchronous motor (IPMSM), a permanent magnet is embedded within a rotor, thereby obtaining a high torque. A generated torque of the IPMSM is expressed by the sum of a magnet torque, which is a torque by a magnet flux, and a reluctance torque which occurs due to a variation of a magnetic resistance. Since the magnet torque is a torque which is proportional to a magnet flux, if a permanent magnet flux varies, the magnet torque will vary, and consequently the generated torque will vary. However, it is known that the permanent magnet within the rotor has such characteristics that the generated flux varies in accordance with the temperature of the permanent magnet itself. Specifically, the generated torque of the IPMSM varies in accordance with a temperature variation of the permanent magnet, and the precision of the actual output torque deteriorates, relative to an output torque reference of the motor.
In addition, if the temperature of the permanent magnet rises by a predetermined degree or more, there occurs a so-called “irreversible demagnetize” in which the magnet flux is not restored even if the temperature falls later. If the state of irreversible demagnetize has occurred, the output torque lowers as a matter of course, and a larger electric current needs to be caused to flow, in order to generate a required torque, leading to deterioration in efficiency of the motor.
In order to cope with the above situation, a technique for detecting the demagnetize state of the flux of the permanent magnet has been developed. In addition, there is known a demagnetize detection technique which was developed by applying a rotational angle sensor-less control technique of a permanent magnet synchronous motor.
In the above technique for detecting the demagnetize state of the flux of the permanent magnet, the demagnetize state of the magnet flux is detected by making use of such a characteristic that the voltage of the motor is proportional to the rotational speed of the motor, with the magnet flux being a proportionality coefficient. In this method, it is necessary that the motor is rotating at a sufficient speed, and the demagnetize state cannot be detected in the halt state.
In addition, in the demagnetize detection technique which was developed by applying a rotational angle sensor-less control technique of a permanent magnet synchronous motor, use is made of such a phenomenon that when a positive/negative bias current has been caused to flow in an estimated d-axis direction at a time of determining the NS polarity of the rotational angle sensor-less control, a magnetic saturation occurs by a composite flux of the flux generated by the bias current and the magnet flux, and the d-axis inductance lowers. Specifically, the demagnetize state is detected, based on the fact that, in the demagnetize state, no magnetic saturation occurs and the d-axis inductance does not lower. However, in this method, the positive/negative bias current needs to be caused to flow successively, and there is a problem that a length of time is needed for detecting the demagnetize state, and moreover it is impossible to detect or estimate the magnet flux amount itself.